Method for Monitoring a Permanent Magnet Motor

ABSTRACT

A test setup for a permanent magnet motor provides a method for identifying symmetry or asymmetry in the magnetic fields of the motor&#39;s permanent magnets. The setup comprises a test circuit that includes a fixed reference node that provides a stable reference to which the motor&#39;s common node can be compared. Observing the waveform of the voltage between the two nodes while the motor is running helps identify an imbalance in the magnetic fields of the motor&#39;s permanent magnets.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 12/321,451 filed on Jan.21, 2009 by the present inventors.

FIELD OF THE INVENTION

The subject invention generally pertains to permanent magnet motors andmore specifically to a setup and method for testing and/or monitoringsuch a motor.

BACKGROUND OF RELATED ART

Permanent magnet motors, such as brushless DC motors and stepper motors,typically include multiple permanent magnets each having a magnet fieldthat interacts with multiple electrical coils to rotate a rotor within astator. Some permanent magnet motors have the magnets on the rotor withthe coils in the stator, and others have the magnets in the stator withthe coils being part of the rotor. In either case, it may be desirableto check the symmetry of the magnetic fields of the permanent magnets toensure proper, smooth operation of the motor. Checking magnetic fieldsymmetry; however, can be difficult to do, particularly after the motoris assembled and operating.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a setup and methodfor testing and/or monitoring a permanent magnet motor.

Another object of some embodiments is to check the symmetry or asymmetryof the magnetic fields provided by a plurality of magnets of a permanentmagnet motor.

Another object of some embodiments is to check the symmetry or asymmetryof the magnetic fields as the motor is energized and running.

Another object of some embodiments is to check the symmetry or asymmetryof a permanent magnet motor's magnetic fields by comparing the voltage,current, and/or waveform of the motor's common node relative to a fixedreference node of an external test circuit.

Another object of some embodiments is to measure the imbalance of amotor's magnetic field, wherein the measured imbalance can be used,depending on rotor design, to monitor rotor conditions due to variationsin mechanical, thermal, or magnetic operating parameters. The imbalancemay be deliberately introduced in the rotor design to allow thismonitoring.

One or more of these and/or other objects of the invention are providedby a motor test setup comprising a test circuit and a permanent magnetmotor. The test circuit includes a fixed reference node that provides areference to which the motor's common node can be compared. Aninstrument sensing the voltage, for example, between the two nodes helpsidentify an imbalance in the magnetic fields of the motor's permanentmagnets.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a method and setup fortesting and/or monitoring a permanent magnet motor.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 schematically illustrates a motor test setup 10 with one exampleof a permanent magnet motor 12 connected to a test circuit 14 fordetecting and evaluating an imbalance or asymmetry in the magnetic fieldof the motor's permanent magnets. The expression, “permanent magnetmotor” means any electromotive rotational machine that includes at leastone permanent magnet having a magnet field interacting with the magnetfield of an electric coil. The permanent magnet could be attached to therotor with the coil being stationary, as shown in the example, or themagnet could be stationary with the coil rotating with the rotor.Examples of a permanent magnet motor include, but are not limited to, abrushless DC motor and a stepper motor. More specific examples of motor12 include TK-85 and TK-106 series motors by PHASE Motion Control ofItaly. For the example illustrated in FIG. 1, motor 12 happens to be abrushless DC motor with a rotor 16 having a plurality of permanentmagnets 18; however, other types of permanent magnet motors with anynumber of magnets and any number of coils can also be tested using themethod described and illustrated here.

Referring to FIG. 1, motor 12 comprises a rotatable shaft 20 supportingrotor 18 within a stator 22. Magnets 18 of rotor 16 are elongate in adirection generally parallel to shaft 20. The magnetic poles of magnets18 run generally radially with the north/south poles alternating fromone magnet to the next. Stator 22 comprises a plurality of coils 24 thatwhen sinusoidally or strategically energized by a polyphase power source26 provide an electrically induced magnetic field that interacts withthe magnet fields of magnets 18 urge rotor 16 to rotate within stator22. The phrase, “strategically energized,” refers to a waveform that issomething other than purely sinusoidal. Such strategic energizing of amotor can be provided, for instance, by a PWM electronic powerconverter. Power source 26 can be a 3-phase system or some otherpolyphase power source (e.g., 2-phase, 4-phase, 5-phase, etc.).

To connect motor 12 to power source 26, the coils of stator 22 include afirst electrical lead 26, a second electrical lead 28 and a thirdelectrical lead 30 that can be wired to power source 26. Also, coils 24share a common node 32. Under normal, non-test operation, node 32 mightbe grounded or left as a floating ground.

To test or monitor the magnetic field imbalance of motor 12, testcircuit 14 comprises a first resistor 34, a second resistor 36 and athird resistor 38 that are wired respectively to leads 26, 28 and 30,whereby power source 26 energizes motor 12 and test circuit 14 in asimilar manner. Resistors 34, 36 and 38 share a reference node 40 thatprovides a reference point to which common node 32 can be compared whenpower source 26 energizes motor 12 and circuit 14. Resistors 34, 36 and38 preferably are of equal resistance with a resistance value suitablefor the power source. An appropriate resistor size for a 480-voltsystem, for example, would be about 1 megohm.

An instrument 42 connected to reference node 40 and common node 32 isable to detect a differential electrical signal 46 between nodes 32 and40, wherein signal 46 reflects how well the plurality of magnet fieldsof magnets 18 are balanced or similar to each other. Instrument 42 canbe any device capable of sensing an electrical difference between nodes32 and 40. Examples of instrument 42 include, but are not limited to, anoscilloscope, a voltmeter, an ammeter, etc. Examples of differentialelectrical signal 46 include, but are not limited to, voltage betweennodes 32 and 40 or electrical current between nodes 32 and 40.

A displayed reading 44 of instrument 42 can show the actual waveform ofsignal 46, wherein signal 46 varies cyclically as rotor 16 rotates, orreading 44 can perhaps be a substantially constant output value such asan RMS value of signal 46. Instrument 42 displaying a sinusoidalwaveform having a peak voltage or current amplitude that issubstantially constant (i.e., each peak has substantially the sameamplitude) can indicate that the magnetic fields of rotor 16 aresubstantially symmetrical. If a generally sinusoidal waveform ofdifferential electrical signal 46 has a peak amplitude that variescyclically as rotor 16 rotates, that could indicate an asymmetry or animbalance in the rotor's magnetic fields.

In the proposed possible case where instrument 42 displays an RMS valueof signal 46, an RMS value of zero could indicate that the rotor'smagnetic fields are substantially symmetrical or balanced. An RMS valueother than zero could indicate appreciable asymmetry with the amplitudeof the RMS value reflecting the magnitude of the asymmetry. The motorsystem can include triplen and other harmonics depending on motor designwhich result in nonzero differential signals and can be used formonitoring. In some cases, a predefined acceptable range of differentialelectrical signal 46 would be where the asymmetry is appreciably greaterthan zero (i.e., not perfectly symmetrical) but less than apredetermined upper limit. In such cases, motor 12 would be consideredacceptable for normal use if signal 46 was within the predefinedacceptable range, but motor 12 would be considered unacceptable fornormal use if signal 46 was beyond the predefined acceptable range(i.e., greater or less than the predefined acceptable range). Theexpression, “normal use” refers to a motor fully functioning as it wasoriginally designed to operate. It should be noted that the terms“symmetrical” and “balanced” are used interchangeably throughout thedescription of the invention. Likewise, the terms, “asymmetrical” and“imbalanced” are also used synonymously.

Although the invention is described with respect to a preferredembodiment, modifications thereto will be apparent to those of ordinaryskill in the art. The scope of the invention, therefore, is to bedetermined by reference to the following claims:

1. A method for monitoring a permanent magnet motor, wherein thepermanent magnet motor includes a stator having a plurality of coilsthat share a common node; a first electrical lead, a second electricallead, and a third electrical lead, all of which are wired to theplurality of coils and are adapted for connection to a polyphase powersource; a rotor having a plurality of permanent magnets providing aplurality of magnetic fields; and a rotatable shaft supporting the rotorwithin the stator; the method comprising: wiring a first resistor, asecond resistor, and a third resistor respectively to the firstelectrical lead, the second electrical lead, and the third electricallead, wherein the first resistor, the second resistor and the thirdresistor share a reference node; energizing the permanent magnet motorvia the polyphase power source; sensing a differential electrical signalbetween the reference node and the common node; detecting an imbalanceof the plurality of magnetic fields based on the differential electricalsignal; and defining an acceptable range of the differential electricalsignal, wherein the differential electrical signal being beyond theacceptable range indicates that the plurality of magnetic fields areappreciably imbalanced.
 2. The method of claim 1, wherein thedifferential electrical signal is a voltage signal.
 3. The method ofclaim 1, wherein the differential electrical signal is a current signal.4. The method of claim 1, wherein the differential electrical signalvaries as the rotor rotates.
 5. The method of claim 1, wherein the firstresistor, the second resistor and the third resistor are ofsubstantially equal resistance.